In recent years, the progress of digital electronic devices has been noticeable. In particular, in mobile electronic devices such as typically cellular phones, digital cameras and clamshell type computers, there is a remarkably increasing demand for higher-frequency actuating signals as well as reduction in size and weight of these devices. Therefore, high-density packaging of electronic parts or wiring circuit boards in these devices is one of the most important technical tasks.
With the recent progress of high-density packaging of electronic parts or wiring circuit boards in electronic devices as well as use of higher-frequency actuating signals therefor, it may be difficult to ensure an adequate distance between the electronic parts that generate noises and the other adjacent parts. For this reason, in order to suppress unnecessary radiation emitted from a microprocessor, LSI or a liquid crystal panel of the electronic devices, electromagnetic interference suppression sheets have been used. In the near electromagnetic filed applications as described above, absorption and reflection phenomena of electromagnetic radiation can be hardly analyzed by a transmission line theory unlike those in the conventionally known far electromagnetic field applications (in which the electromagnetic radiation is in the form of a plane wave) (refer to Osamu HASHIMOTO, “Trend of Wave Absorbers”, Journal of the Institute of Electronics, Information and Communication Engineers, Vol. 86, No. 10, pp. 800-803, October, 2003). For this reason, the electromagnetic interference suppression sheets used in the near electromagnetic field applications have been often designed depending upon experiences of experts only. In recent years, as described in Patent Documents 1 and 2, electromagnetic interference suppression sheets of such a type in which flat magnetic metal particles as soft magnetic particles are blended in a resin, have been used for absorbing electromagnetic radiation in the near electromagnetic magnetic field. The method for evaluating characteristics of an electromagnetic interference (noise) suppression sheet has been standardized by IEC 623333-1 in 2006.
Hitherto, there is described an electromagnetic interference suppressor which comprises, as soft magnetic particles, flat Fe—Al—Si alloy particles having an average particle diameter of 10 μm in an amount of 90% by weight (Patent Document 1). In Patent Document 1, the content of the alloy particles in the electromagnetic interference suppressor is 58.9% by volume when calculated from a density of the alloy particles of 6.9 kg/L and a density of the resin component of 1.1 kg/L with respect to the compositions 1 and 3, and the electromagnetic interference suppressor has a thickness of 1.2 mm.
As the production method, there is described the “method for producing a magnetic sheet in which after a magnetic coating material obtained by dispersing flat magnetic metal particles in a resin and a solvent is applied onto a substrate having a release layer and then dried, the resulting dried coating film is released from the substrate to obtain the magnetic sheet” (Patent Document 2). In Examples of Patent Document 2, there is also described the magnetic shielding sheet having a dried film thickness of 120 μm in which Sendust particles are filled at a maximum filling percentage of 80% by weight (56.0% by volume when calculated from a density of the sendust particles of 6.9 kg/L and a density of the resin of 1.1 kg/L). This indicates that the above method described in Patent Document 2 can realize production of a thinner magnetic sheet as compared to that described in Patent Document 1. It is considered that the thus obtained thin magnetic sheet is more suitable for high-density packaging of electronic parts or wiring circuit boards.
Also, there is disclosed a relationship between a surface electrical resistance and noise suppression of the conductive layer (Nobuyuki HIRATSUKA “Soft Magnetic Materials for Noise Suppression and Applications thereof”, May, 2008).
Further, there is also known a sheet comprising a conductive layer and a magnetic layer which are laminated to each other (Patent Documents 3 to 5 and 10).
In addition, there are known an electromagnetic interference suppression sheet produced by dispersing carbonyl iron particles in a resin (Patent Document 6), and an electromagnetic interference suppression sheet produced by dispersing a conductive carbon and a soft magnetic material in a resin (Patent Documents 7, 8 and 9).